Dynamic Random Access Memory utilizes capacitors to store bits of information within an integrated circuit. Capacitors are also commonly used in other electronic devices.
A capacitor is formed by placing a dielectric material between two electrodes formed from conductive materials. A capacitor's ability to hold electrical charge (i.e., capacitance) is a function of the surface area of the capacitor plates A, the distance between the capacitor plates d (i.e. the physical thickness of the dielectric layer), and the relative dielectric constant or k-value of the dielectric material. The capacitance is given by:
                    C        =                  k          ⁢                                          ⁢                      ɛ            o                    ⁢                      A            d                                              (                  Eqn          .                                          ⁢          1                )            where ∈0 represents the vacuum permittivity.
The dielectric constant is a measure of a material's polarizability. Therefore, the higher the dielectric constant of a material, the more charge the capacitor can hold. Therefore, if the k-value of the dielectric is increased, the area of the capacitor can be decreased and maintain the desired cell capacitance. Reducing the size of capacitors within the device is important for the miniaturization of integrated circuits. This allows the packing of millions (mega-bit (Mb)) or billions (giga-bit (Gb)) of memory cells into a single semiconductor device. The goal is to maintain a large cell capacitance (generally ˜10 to 25 fF) and a low leakage current (generally <10−7 A cm−2). The physical thickness of the dielectric layers in dynamic random-access memory (DRAM) capacitors could not be reduced unlimitedly in order to avoid leakage current caused by tunneling mechanisms which exponentially increases as the thickness of the dielectric layer decreases.
Traditionally, SiO2 has been used as the dielectric material and semiconducting materials (semiconductor-insulator-semiconductor [SIS] cell designs) have been used as the electrodes. The cell capacitance was maintained by increasing the area of the capacitor using very complex capacitor morphologies while also decreasing the thickness of the SiO2 dielectric layer. Increases of the leakage current above the desired specifications have demanded the development of new capacitor geometries, new electrode materials, and new dielectric materials. Cell designs have migrated to metal-insulator-semiconductor (MIS) and now to metal-insulator-metal (MIM) cell designs for higher performance.
One class of high-k dielectric materials possessing the characteristics required for implementation in advanced DRAM capacitors are high-k metal oxide materials. Examples of suitable dielectric materials comprise SiO2, a bilayer of SiO2, and SixNy, SiON, Al2O3, HfO2, HfSiOx, ZrO2, Ta2O5, TiO2, Nb2O5, SrTiO3 (STO), BaSrTiOx (BST), PbZrTiOx (PZT), etc. TiO2 and ZrO2 are two specific examples of metal oxide dielectric materials which display significant promise in terms of serving as a high-k dielectric material for implementation in DRAM capacitors.
Typically, DRAM devices at technology nodes of 80 nm and below use MIM capacitors wherein the electrode materials are metals. These electrode materials generally have higher conductivities than the semiconductor electrode materials, higher work functions, exhibit improved stability over the semiconductor electrode materials, and exhibit reduced depletion effects. The electrode materials must have high conductivity to ensure fast device speeds. Representative examples of electrode materials for MIM capacitors are metals, conductive metal oxides, conductive metal silicides, conductive metal nitrides (i.e. TiN), or combinations thereof. MIM capacitors in these DRAM applications utilize insulating materials having a dielectric constant, or k-value, significantly higher than that of SiO2 (k=3.9). For DRAM capacitors, the goal is to utilize dielectric materials with k values greater than about 40. Such materials are generally classified as high-k materials. Representative examples of high-k materials for MIM capacitors are non-conducting metal oxides, non-conducting metal nitrides, non-conducting metal silicates or combinations thereof. These dielectrics may also include additional dopant materials.
A figure of merit in DRAM technology is the electrical performance of the dielectric material as compared to SiO2 known as the Equivalent Oxide Thickness (EOT). A high-k material's EOT is calculated using a normalized measure of silicon dioxide (SiO2 k=3.9) as a reference, given by:
                    EOT        =                              3.9            k                    ⁢          D                                    (                  Eqn          .                                          ⁢          2                )            where d represents the physical thickness of the capacitor dielectric.
As DRAM technologies scale below the 40 nm technology node manufacturers must reduce the EOT of the high-k dielectric films in MIM capacitors in order to increase charge storage capacity. The goal is to utilize dielectric materials that exhibit an EOT of less than about 0.8 nm while maintaining a physical thickness of about 5-20 nm.
Generally, as the dielectric constant of a material increases, the band gap of the material decreases. For example, the rutile phase of TiO2 has a k-value of about 80 and a band gap of about 3.0 eV while ZrO2 in the tetragonal phase has a k-value of about 43 and a band gap of about 5.8 eV. The low band gap may lead to high leakage current in the device. As a result, without the utilization of countervailing measures, capacitor stacks implementing high-k dielectric materials may experience large leakage currents. High work function electrodes (e.g., electrodes having a work function of greater than 5.0 eV) may be utilized in order to counter the effects of implementing a reduced band gap high-k dielectric layer within the DRAM capacitor. Metals, such as platinum, gold, ruthenium, and ruthenium oxide are examples of high work function electrode materials suitable for inhibiting device leakage in a DRAM capacitor having a high-k dielectric layer. The noble metal systems, however, are prohibitively expensive when employed in a mass production context. Moreover, electrodes fabricated from noble metals often suffer from poor manufacturing qualities, such as surface roughness, poor adhesion, and form a contamination risk in the fab.
Leakage current in capacitor dielectric materials can be due to Schottky emission, Frenkel-Poole defects (e.g. oxygen vacancies (Vox) or grain boundaries), or tunneling which may include direct tunneling, Fowler-Nordheim tunneling, or both. Schottky emission, also called thermionic emission, is a common mechanism and is the thermally activated flow of charge over an energy barrier whereby the effective barrier height of a MIM capacitor controls leakage current. The nominal barrier height is a function of the difference between the work function of the electrode and the electron affinity of the dielectric. The electron affinity of a dielectric is closely related to the conduction band offset of the dielectric. The Schottky emission behavior of a dielectric layer is generally determined by the properties of the dielectric/electrode interface. Frenkel-Poole emission allows the conduction of charges through a dielectric layer through the interaction with defect sites such as vacancies, grain boundaries, and the like. As such, the Frenkel-Poole emission behavior of a dielectric layer is generally determined by the dielectric layer's bulk properties. “Direct tunneling” as used herein refers to electrons tunneling directly through the forbidden energy barrier of a dielectric layer. If the dielectric is very thin, the electrons may tunnel through the entire thickness of the layer. “Fowler-Nordheim tunneling” as used herein refers to electrons tunneling initially from the inversion layer of a conductor or semiconductor to the dielectric's conduction band, and then across the barrier. Without being restricted to any particular theory, Fowler-Nordheim tunneling may allow conduction of charges through a dielectric layer without any intermediary interaction (e.g., with defects in the dielectric). Fowler-Nordheim tunneling becomes a significant effect as dielectric thickness and the strength of the imposed electric field are increased. Leakage due to tunneling has been a primary motivation for the substitution of high-k dielectric materials where low-k dielectrics (e.g., SiO2) were previously used. The use of high-k materials allows the use of a physically thicker dielectric layer (i.e., too thick for tunneling) while maintaining the required capacitance (see Eqn 1 above).
The mechanisms for charge transport discussed above suggest that there are several parameters that influence the leakage current across the metal-dielectric interface. Examples of the parameters include physical thickness of the dielectric material, the band gap of the dielectric material, the work function of the metal, the Schottky barrier height (SBH) between the metal and the dielectric material, etc. The SBH has been found to be influenced by many variables such as the composition of the metal and the dielectric, doping levels, defect densities, processing conditions, etc. However, as discussed in the review article by Tung (Materials and Science Engineering, R 35, (2001), pgs. 1-138) which is herein incorporated by reference, in an ideal case, the SBH, Φ0B,n, is the difference between the work function, φm, of the metal and the electron affinity, Xs, of the dielectric as illustrated in Eqn. 3 based on the Schottky-Mott theory.ΦB,n0=φm−Xs  (Eqn. 3)
Eqn. 3 implies that the variation of the SBH with respect to the work function, S101, of the metal should be 1 for a given dielectric material as illustrated in Eqn. 4. This quantity, SΦ, is defined as the interface behavior parameter or simply the S-parameter. Experimentally, this has been found to be less than 1, indicating that there are additional factors that influence the SBH.
As discussed previously, one of the methods for reducing the leakage current in capacitors is to use metal electrode materials with a high work function. However, if the S-parameter of the dielectric is close to 0, then the use of metal electrode materials with a high work function will not improve the leakage current.